The present invention relates generally to a system for detecting the motion of a vehicle. More particularly, the invention concerns a motion detector that provides an indication of various states or conditions of motion of a railway vehicle by monitoring both vibrations within the vehicle and the acceleration thereof.
In railway systems, such as those employing locomotive-drawn trains, it is difficult for the engineer or other operator to reliably be apprised of the state of motion of vehicles that are located remotely from him. For example, when starting a train from a stopped condition, it is particularly difficult for the train driver to known when the driving force of the locomotive has propagated through the interconnected cars and accelerated the last vehicle into motion. Conversely, when coming to a stop, it is difficult for the driver to know when the last car has been decelerated to a standstill. Knowledge of these conditions of motion of the last vehicle is extremely useful to the driver in controlling operation of the train. Since, in the normal operation of a train, it is not uncommon for the train to reverse directions at low speeds, it is important that the driver know when the last car has undergone the desired change in motion as, for example, from reverse to forward.
When the train is in full motion, individual vehicles and groups of vehicles undergo changes in velocity as uphill and downhill grades are negotiated. These changes occur at differing intervals and affect how the driver controls the train. The condition in which one car is being "pulled" by an adjacent car is referred to as "draft", while the condition in which a car is "pushed" by an adjacent car is referred to as "buff". Reliable information concerning these conditions would be very useful to the train driver.
Heretofore, no means have been developed for reliably and continuously detecting the motion of remote rail vehicles. The use of Doppler effect radar has been proposed as one such means. While potentially enabling precise velocity measurement, a Doppler effect radar suffers from a number of practical problems when utilized as a rear-of-train motion detector. For antennas of reasonable size, the radiation pattern is so broad that passing trains or other vehicles produce large spurious outputs. Even if a more highly directional antenna were possible, passing trains would still yield spurious outputs on curved sections of track. It is also difficult to adequately shield a microwave antenna of small dimensions from the effects of rain, snow, and ice so that transmission and reception are not seriously compromised. Lastly, transmitters for ten gigahertz, which are normally used in this application, draw significant power. This is particularly disadvantageous where it is desired to position the velocity detector on a vehicle where little power is available.
The sensing of vibrations is a generally known means for detecting motion. For example, some commercially available accelerometers employ the piezoelectric effect to generate signals that are indicative of vibration. However, a number of problems are encountered in attempting to provide a device that relies upon vibrations within a vehicle to indicate differing states of motion. First, there is a significant problem of adequately discriminating between vibrations resulting from movement of the subject vehicle and those resulting from other mechanisms such as a passing train. If the vibrations resulting from a passing train are to be rejected, sensitivity of the sensor must be set so low that vibrations of the subject car may also be rejected. This is particularly problematic, since the sensor would frequently indicate a "stop" condition while the subject car is indeed moving, such as at a low speed on smooth rail. Conversely, a high-sensitivity sensor that can reliably detect even slow motion on smooth rail will almost invariably also detect passing trains and similar sources of vibration external to the subject vehicle.
Secondly, reliance upon vibration sensing presents difficulties when employed on a vehicle equipped with an onboard engine, such as a diesel engine for a refrigerated car. In such an application, it is difficult to yield a reliable change in output signal corresponding to vehicle motion since the onboard engine vibration is of such amplitude and frequency spectrum as to effectively mask the motion-indicating vibrations.
Thirdly, given the inherent nondirectionality of vibration sensing, it is difficult to differentiate between forward and reverse motion. As noted above, this indication of direction can be very useful to the train driver.